Now it's possible to specify the desired trybots for perf
tries, e.g.:

tools/try_perf.py --linux64_haswell octane sunspider

BUG=chromium:478460
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1114913002

Cr-Commit-Position: refs/heads/master@{#28142}